Hi. I'm working on a futuristic, sci-fi type novel, where much of the technology is based on, and a fair bit of the plot ties to nanotechnology, genetic modification, etc. In general, I try to keep things relatively realistic and pragmatic. I'm trying hard not to veer off into sci-fi fantasy, as tempting as that can be for the sake of convenience. Basically certain members of a kind of futuristic aristocracy age very slowly due to a combination of, again, nanotech, eugenics and genetic modification. As such, their biology and appearance doesn't generally reflect their chronological age. My question is: In a relatively realistic sci-fi setting, say, 200 years in the future, what are the realistic limits of delaying aging to you? Or rather, at what point does it become unrealistic? Someone who looks 30 biologically, but is chronologically 50, 60, 80, 100, indefinite, etc? Not really looking for validation or whatever. More so trying to gauge the parameters of suspending disbelief when it comes to this issue. And of course, any and all comments, suggestions, etc. are welcome. Thank you.